Brief Communication: Outburst Floods Triggered By
This is a repository copy of Brief Communication: Outburst floods triggered by periodicdrainage of subglacial lakes, Isunguata Sermia, West Greenland.White Rose Research Online URL for this n: Submitted VersionArticle:Livingstone, S.J. orcid.org/0000-0002-7240-5037, Sole, A.J.orcid.org/0000-0001-5290-8967, Storrar, R.D. orcid.org/0000-0003-4738-0082 et al. (3more authors) (Submitted: 2019) Brief Communication: Outburst floods triggered byperiodic drainage of subglacial lakes, Isunguata Sermia, West Greenland. The CryosphereDiscussions. ISSN 1994-0432 eThis article is distributed under the terms of the Creative Commons Attribution (CC BY) licence. This licenceallows you to distribute, remix, tweak, and build upon the work, even commercially, as long as you credit theauthors for the original work. More information and the full terms of the licence If you consider content in White Rose Research Online to be in breach of UK law, please notify us byemailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal terose.ac.uk/
https://doi.org/10.5194/tc-2019-137Preprint. Discussion started: 3 July 2019c Author(s) 2019. CC BY 4.0 License.Brief Communication: Outburst floods triggered by periodicdrainage of subglacial lakes, Isunguata Sermia, WestGreenland5Stephen J. Livingstone1, Andrew J. Sole1, Robert D. Storrar2, Devin Harrison3, Neil Ross3,Jade Bowling41Department of Geography, University of Sheffield, Sheffield, UK2Department of the Natural and Built Environment, Sheffield Hallam University, UK3School of Geography, Politics and Sociology, Newcastle University, Newcastle upon Tyne, UK410Lancaster Environment Centre, Lancaster University, Lancaster, UKCorrespondence to: Stephen J. Livingstone (s.j.livingstone@sheffield.ac.uk)Abstract (100 words)15We report three active subglacial lakes within 2 km of the lateral margin of Isunguata Sermia, West Greenland,identified by differencing time-stamped ArcticDEM strips. Each lake underwent one drainage-refill event between2009 and 2017, with two lakes draining in 1 month during August 2014 and August 2015, and all threecharacterised by 2-3-year refill periods. The 2015 drainage flooded the foreland aggrading 8 m of the proglacialchannel, confirming the ice-surface elevation anomalies as subglacial water bodies and demonstrating howsubglacial lake drainages can significantly modify proglacial environments. These subglacial lakes offeraccessible targets for future geophysical investigations and exploration.201. Introduction25303540Meltwater beneath the Greenland Ice Sheet is sourced from geothermal and frictional melting, and via the inputof surface meltwater through englacial pathways. This meltwater drains towards the ice sheet margin through acomplex network of inefficient and efficient drainage routes (Davison et al., 2019). Spatial and temporal variationsin drainage structure are controlled by the hydraulic gradient and meltwater flux. Steeper hydraulic gradients andhigher meltwater fluxes close to the ice margin lead to greater ice melt rates and promote the formation of efficientchannels, which can extend up to 40 km inland and evolve on seasonal timescales in response to surface meltwaterinputs (Chandler et al., 2013). Shallow hydraulic gradients and lower meltwater fluxes dominated by subglacialmeltwater sources tend to be associated with more inefficient drainage configurations further inland (Doyle et al.,2014).Storage of water in firn (Forster et al., 2013), damaged englacial ice (Kendrick et al., 2018) and both supraglacial(Selmes et al., 2011) and subglacial lakes (Palmer et al., 2013; Oswald et al., 2018; Bowling et al., 2019) candelay the drainage of meltwater through the ice sheet to the ocean, while the rapid drainage of stored water canoverwhelm the drainage system and perturb ice flow (e.g. Das et al., 2008). Storage and drainage of supraglaciallakes have been well-documented (e.g. Selmes et al., 2011), but the volume of water stored subglacially, and thelakes’ residence times and wider influence on the subglacial hydrological system and ice flow is poorlyunderstood. Although expected to be a less significant component of the hydrological system compared withAntarctica (e.g. Siegfried & Fricker, 2018) due to steeper hydraulic gradients, dominance of surface inputs andmore efficient subglacial water routing, 1000s of subglacial lakes have been predicted and over 50 identifiedbeneath the Greenland Ice Sheet (Livingstone et al., 2013; Bowling et al., 2019). This includes stable lakes abovethe Equilibrium Line Altitude (ELA) but away from the interior, hydrologically active lakes near the ELArecharged by surface meltwater, and small seasonally active lakes below the ELA which form during winter anddrain during the melt season (Palmer et al., 2013, 2015; Howat et al., 2013; Willis et al., 2015; Chu et al., 2016;Oswald et al., 2018; Bowling et al., 2019).1
https://doi.org/10.5194/tc-2019-137Preprint. Discussion started: 3 July 2019c Author(s) 2019. CC BY 4.0 License.455055Whilst seasonal water storage is thought to be common below the ELA (e.g. Chu et al., 2016; Kendrick et al.,2018), longer term subglacial lake storage is thought unlikely due to the development of efficient channels andassociated increase in hydrological connectivity each melt season. In this paper we acquired multi-temporalArcticDEM Digital Surface Models (DSMs) (Noh & Howat, 2015) and Landsat 7 and 8 satellite imagery between2009 and 2017 to identify three active subglacial lakes on a reverse bed-slope beneath Isunguata Sermia, WestGreenland (67 10’ N, 50 12’W) (Fig. 1). The ArcticDEM DSMs were generated from high-resolution satelliteimagery and have a spatial resolution of 2 m and internal accuracy of 0.2 m. Each of the 52 DSMs acquired overthe time period were corrected against filtered IceSAT altimetry data using the metadata provided (Dai & Howat,2017). Change in Normalised Difference Water Index (NDWI) to identify flooding of the proglacial zone wascalculated using top-of-atmosphere corrected Landsat green (band 3) and near-infrared (band 5) bands and theformula: NDWI (band 3 – band 5)/(band 3 band 5).2. Observations606570758085Yearly ice-surface elevation change was determined from 2009 to 2017 by differencing the multi-temporalArcticDEM DSMs. This revealed three distinctive quasi-circular regions, hereafter referred to as ‘anomalies’, allwithin 2 km of the lateral margin of Isunguata Sermia, that were characterised by periods of subsidence followedby uplift (Fig. 1). Timeseries of relative elevation change for each anomaly were calculated from the DSMs bysubtracting the mean ice-surface elevation of the anomaly from the mean elevation of a 500 m buffer around it(Fig. 2). This approach was used to isolate the dynamic effect and to remove the influence of systematic verticaland horizontal offsets (of up to 3-5 m) between DSMs. Anomaly 1, located 5 km from the terminus of IsunguataSermia, formed a 0.93 km2 depression between 19th August 2010 and 3rd August 2011 with a mean depth of 5 mand maximum depth of 17 m. The ice-surface then rose 1 m by November 2011 before recovering back to its 2010elevation by February 2013. Anomaly 2, about 1 km further up ice, formed a 0.88 km2 depression between 2ndAugust 2015 and 21st September 2015, with a mean depth of 13 m and maximum depth of 30 m. It has since risen9 m between 2015 and 2017. Anomaly 3, which is just up-ice from anomaly 2 and 9 km from the terminus,formed a 0.67 km2 depression between 17th August 2014 and 19th September 2014, with a mean depth of 4 m andmaximum depth of 14 m, before the surface rose 3 m between 2014 and 2017. Surface structural features indicativeof localised ice fracture such as crescentic crevasses are not apparent in any of the depressions.Landsat 8 OLI satellite images acquired before and after the 2015 ice-surface subsidence (anomaly 2) reveal amajor change in the 1.8 km wide proglacial braided river system (Fig. 3). On 15 th July 2015 the river plain ischaracterised by a single channel emanating from the front of Isunguata Sermia, that then bifurcates down-riverinto multiple braids and intervening bars (Fig. 3a). Dry areas above the water level are demarcated by a sharpchange in colour, with wetted areas darker and dry areas lighter. Using this demarcation, a major flood plaindirectly in-front of the main portal, which causes the river emanating from the glacier to divert northwards andthen westwards, is identified. On the basis of a qualitative change from light to dark, on the 25th August 2015, anda quantified positive change in NDWI of up to 0.23 (mean: 0.09) between July and August, the dry areas (barsand floodplain) became inundated by water and the braided river system re-organised (Fig. 3b-c). DifferencingArcticDEM DSMs of the proglacial area before (4th May 2015) and after (21st September 2015) the ice-surfaceelevation change associated with anomaly 2 reveals 3 m of mean net sediment aggradation across a 5 km stretchof the main proglacial channel (Fig. 3d). Aggradation was up to 8 m close to the outlet and declined to 1 m 5 kmfrom the glacier terminus.3. Discussion9095We identify three anomalies on Isunguata Sermia characterised by localised ice-surface elevation changes, whichwe interpret as subglacial lake drainage and filling (Fig. 1). Confirmation of a subglacial lake origin is providedby flooding of the proglacial outwash plain in August 2015, which coincided with the timing of ice-surfaceelevation anomaly 2, evidencing the release of meltwater (Fig. 3). This inundation (wetting) of the flood plain isnot replicated at the nearby Leverett-Russell Glacier (Fig. 3b), ruling out a common external forcing (e.g. heavyrainfall). All three subglacial lakes are located under 325-400 m thick (Lindbäck et al., 2014), warm-based ice ona reverse gradient slope (15 m per km); the reverse slope may be trapping the water causing the lakes to form.Although subglacial hydrological analysis (e.g. Lindbäck et al., 2014; Chu et al., 2016) does not produce hydraulic2
https://doi.org/10.5194/tc-2019-137Preprint. Discussion started: 3 July 2019c Author(s) 2019. CC BY 4.0 License.minima in the locations where we identify lakes, this may be a result of the limited and relatively poor-qualityairborne radar ice-thickness measurements across the thin, near-marginal area of Isunguata Sermia.100105110115120125130135140145The three subglacial lakes each underwent one drainage event over the 8-year data period (Fig. 2). Differencingof the DSMs either side of the drainage events, over the area of the ice-surface anomalies, gives total lake volumechanges of 6.5 0.52 x 106 m3, 1.3 0.05 x 107 m3 and 3.5 0.38 x 106 m3 for anomalies 1-3 respectively.Drainage of Subglacial Lake 2 in 2015 and Lake 3 in 2014 were both triggered in August and drained in 1 month,which is consistent with other larger subglacial lake drainage events identified in Greenland (Howat et al., 2014;Palmer et al., 2015; Willis et al., 2015), but contrasts with the longer (months to years) drainage period of thosein Antarctica (e.g. Siegfried & Fricker, 2018). If the vertical displacement of the ice-surface is assumed to beequivalent to the depth of the subglacial lake, this gives a mean minimum discharge of 6.5 m3 s-1 for SubglacialLake 2, which is the largest and best-constrained by available DSM and satellite imagery in this study.Lake recharge is on the scale of a few years, and it is noticeable that the largest subglacial lake drainage event(Lake 2) subsequently refilled at the fastest rate ( 5 m uplift yr-1), while the smallest drainage event (Lake 3) isfilling at the slowest rate ( 1 m uplift yr-1). The lakes are at the lower end of the ablation zone and therefore likelyto be dominated by upstream surface meltwater inputs and the seasonal melt signal (Davison et al., 2019). Despitethis, lake drainage is not associated with high-melt years (e.g. the 2011 drainage event coincided with a low meltyear) and recharge rates were similar over winter and summer. For example, the ice surface above Lake 2 rose 3m over a 5-month period between September 2015 and February 2016 immediately following drainage, but thenrose an equivalent height over the next 6-month period between February and August 2016. This may be partiallydue to faster initial recovery, but also implies that the lake is able to capture significant volumes of water overwinter. All three lakes exhibited quiescent periods of extended high-stand, which might occur when water flowinto the lake is balanced by outflow, and suggests an external threshold controlling lake drainage initiation.Although in close proximity, drainage of an upstream lake does not trigger a cascade of drainage in downstreamlakes. In addition, the filling of Lake 3 did not limit recharge of Lake 2 just downstream (Fig. 2). This suggeststhat the lakes are not hydraulically well connected, consistent with subglacial hydraulic modelling indicating themain subglacial drainage axis is just to the north of the two upstream subglacial lakes (Fig. 3a). Both the 2014 and2015 drainage events were initiated in August at a time when drainage system connectivity is envisaged to be highand water preferentially drains towards efficient channels along a hydraulic gradient (Davison et al., 2019). Thus,rapid drainage could be a response to lakes infrequently connecting with the main subglacial channel.The August 2015 subglacial lake drainage event flooded the foreland and resulted in substantial net ice-proximalsediment aggradation (7.5 x 106 m3) of the outwash plain (Fig. 3). Deposition was greatest in the main channel,with up to 8 m of net aggradation close to the outlet diminishing to 1 m 5 km from the terminus. This nearmargin pattern of aggradation is consistent with the geomorphic impact of jökulhlaups observed in Iceland (e.g.Dunning et al., 2013) and demonstrates the potential of episodic subglacial lake drainage events to erode thesubglacial bed and modify the proglacial environment. Given the subglacial lake is located just 8 km from theglacier terminus the subglacial erosion necessary to produce the sediment volume deposited on the foreland isequivalent to a 10 m deep and 100 m wide channel cut into the bed. The restricted pattern of deposition at thesouthern end of the glacier terminus suggests that the subglacial drainage event was at least partially focused intoa channel rather than an unconstrained sheet flood.Although the presence or absence of sediments in these lakes has yet to be tested, these three subglacial lakespresent an extremely accessible target for future geophysical characterisation and active lake exploration. Icesurface elevation changes suggest the lakes are at least 14-30 m deep and have minimum volumes of 3.5-13 x 106m3. Ice cover is relatively thin (325-450 m) and the lakes are clustered and in close proximity to the ice margin( 2 km), road ( 5 km) and key logistical support including a major airport (Kangerlussuaq). Key questions thatcould be addressed through detailed investigation of these lakes include: what triggers subglacial lake drainageand how does drainage evolve downstream? How do lakes interact with other components of the subglacialdrainage system? What geomorphological and sedimentological signatures of similar drainage events might berecorded in the proglacial area?4. Conclusions3
https://doi.org/10.5194/tc-2019-137Preprint. Discussion started: 3 July 2019c Author(s) 2019. CC BY 4.0 License.150155160Using multi-temporal ArcticDEM DSM and satellite imagery, we identify three active subglacial lakes 10 kmfrom the terminus of Isunguata Sermia. The lakes are characterised by periods of relative inactivity punctuated byrapid drainage ( 1 month) and then slow recharge (a few years). The most recent drainage event in 2015 floodedthe outwash plain resulting in net ice-marginal sediment aggradation that was greatest closest to the outflow portaland thinned downstream. This work demonstrates the potential for subglacial lakes to exist in the lower ablationzone close to the ice margin, where subglacial hydrology is dominated by surface seasonal meltwater inputs andefficient channelized drainage. The lakes appear to be only weakly connected to the main subglacial channel axisand drainage may be controlled by the ability of this channel to occasionally capture water from its surroundings.The 2015 subglacial lake drainage event had a significant subglacial and proglacial geomorphic impact, includingsubstantial erosion of sediment from beneath Isunguata Sermia and substantial aggradation of sediment in theproglacial outwash plain close to the terminus. Detailed geophysical studies across and downstream of these lakeswould provide insight into the conditions causing the lakes to form and drain, the resultant geomorphic imprintand the depositional archive of these lake environments. Crucially, these subglacial lakes may be the mostaccessible in the world due to their setting beneath thin ice close to the lateral margin of the glacier and the existinginfrastructure and logistical set-up of the region.Author contributions. A. Sole and R. Storrar initially identified the subglacial lakes. S. Livingstone, A. Sole andD. Harrison processed and analysed the data. S. Livingstone wrote the paper with input and ideas from all coauthors.165Acknowledgements. DEMs provided by the Polar Geospatial Center under NSF-OPP awards 1043681, 1559691,and 1542736. This work was supported by Ph.D. studentships awarded to D. Harrison through the IAPETUSNatural Environmental Research Council Doctoral Training Partnership (NE/L002590/1) and J. Bowlingthrough the ENVISION Natural Environmental Research Council Doctoral Training Partnership(EAA6583/3152).170ReferencesBowling, J., Livingstone, S.J., Sole, A.J. and Chu, W. Distribution and dynamics of Greenland subglacial lakes.Nature Communications, 10:2810, doi.10.1038/s41467-019-10821-w, 2019.175Chandler, D.M., Wadham, J.L., Lis, G.P., Cowton, T., Sole, A., Bartholomew, I., Telling, J., Nienow, P.,Bagshaw, E.B., Mair, D. and Vinen, S. Evolution of the subglacial drainage system beneath the Greenland IceSheet revealed by tracers. Nature Geoscience, 6, 195. doi.10.1038/ngeo1737, 2013.Chu, W., Schroeder, D.M., Seroussi, H., Creyts, T.T., Palmer, S.J. and Bell, R.E. Extensive winter subglacialwater storage beneath the Greenland Ice Sheet. Geophysical Research Letters, 43, doi.10.1002/2016gl071538,2016.180Dai, C. and Howat, I.M. Measuring lava flows with ArcticDEM: Application to the 2012–2013 eruption ofTolbachik, Kamchatka. Geophysical Research Letters, 44, doi.10.1002/2017gl075920, 2017.Das, S.B., Joughin, I., Behn, M.D., Howat, I.M., King, M.A., Lizarralde, D. and Bhatia, M.P. Fracture propagationto the base of the Greenland Ice Sheet during supraglacial lake drainage. Science, 320, 778-781,doi.10.1126/science.1153360, 2008.185190Davison, B.J., Sole, A.J., Livingstone, S.J., Cowton, T.R. and Nienow, P.W. The influence of hydrology on thedynamics of land-terminating sectors of the Greenland Ice Sheet. Frontiers in Earth Science, 7,doi.10.3389/feart.2019.00010, 2019.Doyle, S.H., Hubbard, A., Fitzpatrick, A.A., Van As, D., Mikkelsen, A.B., Pettersson, R. and Hubbard, B.Persistent flow acceleration within the interior of the Greenland ice sheet. Geophysical Research Letters, 41, 899905, doi.10.1002/2013gl058933, 2014.4
https://doi.org/10.5194/tc-2019-137Preprint. Discussion started: 3 July 2019c Author(s) 2019. CC BY 4.0 License.Dunning, S.A., Large, A.R., Russell, A.J., Roberts, M.J., Duller, R., Woodward, J., Mériaux, A.S., Tweed, F.S.and Lim, M. The role of multiple glacier outburst floods in proglacial landscape evolution: The 2010Eyja
delay the drainage of meltwater through the ice sheet to the ocean, while the rapid drainage of stored water can overwhelm the drainage system and perturb ice flow (e.g. Das et al., 2008). Storage and drainage of supraglacial 35 lakes have been well-documented (e.g. Selmes et al., 20
GLACIAL LAKE OUTBURST FLOOD (GLOF) ‘REDUCING RISKS AND ENSURING PREPAREDNESS 1 Information sharing and cooperation on cryospheric research in the
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